Table of Contents · 6 sections
Antibiotic classification is one of the core topics in pharmacology, essential for clinical practice. Questions about antibiotic groups, mechanisms of action, and side effects appear on every exam—from 3rd-year pharmacology to the accreditation exam. This article provides a comprehensive systematization with tables, clinical indications, and mnemonics.
Principles of Antibiotic Classification
Antibiotics are classified according to several criteria:
- By mechanism of action — the most clinically significant classification
- By spectrum — narrow- and broad-spectrum
- By type of action — bactericidal (kill) and bacteriostatic (halt growth)
- By chemical structure — beta-lactams, macrolides, aminoglycosides, etc.
Classification by Mechanism of Action
1. Inhibition of Cell Wall Synthesis
These are bactericidal drugs — they kill bacteria by disrupting peptidoglycan synthesis:
| Group | Examples | Target | Spectrum |
|---|---|---|---|
| Penicillins | Amoxicillin, ampicillin, oxacillin, piperacillin | PBPs (penicillin-binding proteins) | Gram+ (oxacillin), broad (amoxicillin + clavulanate) |
| Cephalosporins | I — cefazolin; II — cefuroxime; III — ceftriaxone, ceftazidime; IV — cefepime; V — ceftaroline | PBP | Expands towards Gram-negative across generations |
| Carbapenems | Meropenem, imipenem, ertapenem | PBP | Ultrabroad (reserve) |
| Glycopeptides | Vancomycin, teicoplanin | D-Ala-D-Ala (terminal dipeptide) | Gram+ (including MRSA) |
2. Inhibition of Protein Synthesis
| Ribosomal subunit | Group | Examples | Type of action |
|---|---|---|---|
| 30S | Aminoglycosides | Gentamicin, amikacin, tobramycin | Bactericidal |
| 30S | Tetracyclines | Doxycycline, tigecycline | Bacteriostatic |
| 50S | Macrolides | Azithromycin, clarithromycin, erythromycin | Bacteriostatic |
| 50S | Lincosamides | Clindamycin | Bacteriostatic |
| 50S | Chloramphenicol | Levomycetin | Bacteriostatic |
| 50S | Oxazolidinones | Linezolid | Bacteriostatic |
3. Inhibition of Nucleic Acid Synthesis
- Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) — inhibit DNA gyrase and topoisomerase IV. Bactericidal, broad-spectrum
- Rifampicin — inhibits RNA polymerase. Primary drug for tuberculosis treatment
- Metronidazole — damages DNA in anaerobes. Drug of choice for anaerobic infections
4. Antimetabolites
Sulfonamides + trimethoprim (co-trimoxazole, biseptol) — disrupt folic acid synthesis at two sequential steps. Bactericidal combination.
Side Effects of Main Groups
| Group | Characteristic side effects | Mnemonic |
|---|---|---|
| Penicillins | Allergic reactions (up to anaphylaxis), diarrhea | "Penicillin = Cross-allergy with cephalosporins (1–10%)" |
| Aminoglycosides | Nephrotoxicity, ototoxicity | "Amino-Deafer-Nephro" |
| Fluoroquinolones | Tendinitis / tendon rupture, photosensitization, QT prolongation | "Fluoroquinolones tear the Achilles" |
| Tetracyclines | Photosensitization, tooth discoloration in children, bone growth impairment | Contraindicated in children under 8 years of age and pregnant women |
| Macrolides | Hepatotoxicity (erythromycin), QT prolongation, CYP3A4 inhibition | Azithromycin is the least hepatotoxic |
| Vancomycin | "Red man" syndrome (with rapid IV infusion), nephrotoxicity | Infuse slowly (>60 minutes)! |
| Chloramphenicol | Aplastic anemia (rare but fatal), "gray baby syndrome" in newborns | Hardly used due to toxicity |
Clinical antibiotic selection
Main clinical situations and first-line drugs:
- Community-acquired pneumonia (non-severe) — amoxicillin or a macrolide (azithromycin). Read more: Diagnosis and treatment of pneumonia
- Urinary tract infections (uncomplicated) — fosfomycin (single dose), nitrofurantoin
- Bacterial meningitis — ceftriaxone + ampicillin (in newborns and patients >50 years)
- MRSA infection — vancomycin or linezolid
- Anaerobic infection — metronidazole + 3rd generation cephalosporin
- H. pylori eradication — amoxicillin + clarithromycin + PPI. Read more: Peptic ulcer disease
Resistance: ESKAPE pathogens
Group of the most dangerous resistant pathogens:
- Enterococcus faecium (VRE)
- Staphylococcus aureus (MRSA)
- Klebsiella pneumoniae (ESBL, carbapenem-resistant)
- Acinetobacter baumannii (pandrug-resistant)
- Pseudomonas aeruginosa (MDR)
- Enterobacter spp. (AmpC, carbapenem-resistant)
Frequently asked questions
What is the difference between bactericidal and bacteriostatic antibiotics?
Bactericidal antibiotics kill bacteria (beta-lactams, aminoglycosides, fluoroquinolones). Bacteriostatic antibiotics stop their multiplication (macrolides, tetracyclines, chloramphenicol). Bactericidal agents are preferred for immunocompromised patients.
Why shouldn't tetracyclines be prescribed to children?
Tetracyclines form chelate complexes with calcium and deposit in tooth enamel (yellow discoloration) and bone growth plates. Contraindicated under 8 years of age and in pregnant women.
What is empirical antibiotic therapy?
Prescribing an antibiotic before culture results are available, based on the most probable causative agent and local resistance data. After receiving the antibiogram, treatment is adjusted (de-escalation).
Memorize antibiotics via interactive pharmacology lessons. Related topics: Gram stain and bacterial classification, NSAIDs, CURB-65 calculator.